It’s not difficult to understand at all.
I can see this from the point of view of those from other EU nations. If, for example, you are French and like the idea of huge sums of foreign taxpayers’ dosh being shovelled into your inefficient farming industry I can see your point. If you are Romanian or Bulgarian it must be nice to see similarly large sums come your country’s way under the “infrastructure and cohesion” scheme (whilst being able to settle in, say, the UK, where you can earn up to ten times the amount you could at home). That’s fair enough.
As well as this, I seem to recall that quite a number of UK Remainers are unhappy with the direction the EU is travelling but want to remain in nonetheless. (Like Mrs May they could be called “reluctant Remainers”).They see the advantages outweighing the disadvantages. I don’t quite see the up side for the UK though (and even if I did, no “up side” is worth sacrificing our sovereignty for).
The difficulty that the Remainers (both reluctant and eager) would have faced had we chosen to remain is that there will certainly come a time when the direction in which the EU is travelling takes most of them them to a situation where almost all of them will be unhappy. By that time it will be too late to opt out of the various “concessions” the UK currently enjoys. They will be withdrawn as sure as eggs is eggs (because that’s how the EU works). Then it will be too late. The direction the EU is taking is to a Federation ruled from Brussels. The Euromaniacs may suffer of setback or two due to “popularist” votes from time to time but, as has been witnessed, they will not allow a trifling thing such as the wishes of the people get in their way.